Electromagnetic sound-transmission apparatus



June 1946. R. GOLDSCHMIDT ELECTRQ-MAGNETIQ SOUND-TRANSMISSION APPARATUS Filed Oct. 6, 1942 Fig. 5.

Fig. 4.

Patented June is, 1946 UNITED Y STATES PATENT OFFICE zaozsaz mlncraomonarrc sown-uninsure- SION mm'rus A amen Goldschmidt, Sheilicld, England, assignmto New York Radio Patents Corporation, a corporation oi Application October 6, 1942, Serial No. 460,950

Y In Great October 23, 1941 This invention relates to improvements in electromagnetic sound transmission devices especially foruse in hearing aids and other. sound transducers.

Although the invention will be described with special reference to a hearing aid device, it will be evident that the novel magnetic control of the sound output will have similar use and application in, other sound transducers such as telephones, loudspeakers, etc.

When deafness is due to a defect of the middle ear, sound can be transmitted to the auditory nerves through the skull. This system, known as the bone-conduction method, empioysa microphone for transforming sound waves into alterdevice containing a flexible member, for example a reed, adapted to be set in vibration. by the current and to transmit the mechanical vibrations thus produced to the skull, through the skin. I

This conduction of the vibrations is handi- Claims. (cure-107) 2'. v w an electro-magnetic sound-translating device adapted to provide a measure of magnetic ampliflcatlon.

These and other features of this invention will be apparent from the following description of embodiments thereot, given by way of example, with is hating electric current, and an electro-magnetic capped by the compliance of the skin and the tissues underlying it, which is considerable compared with that of the massive bones underneath and with their high impedance for high-frequency components of thesounds. Consequently these components are filtered out in various degrees,

frequencies are unduly amplified and others pres-- tically suppressed.

Although resonances and electrical amplification may be used to create a sense of loudness, the reproduction may be so distorted as to make speech intelligible only with difilculty, particu: larly if the pitch of the voice is not in harmony with the basic resonances of the apparatus Furthermore, distorted music is, of course, very unsatisfactory.

An object of the present invention is to overcome these disadvantages of known bone-conduction deaf-aid systems by substantially reducing the distortion of the vibrations affecting the auditory nerves and by increasing their loudness.

A further object of this invention is to provide electrc-magnetic sound-transmission apparatus,

reference to the accompanying drawing, in which: Fig. 1 shows diagrammatically the improved deafaid apparatus in use,

Fig. 2 shows in section to a larger scale a modified form of armature applicable to the apparatus shown'in Fig. l. i

Fig. 3 is an elevation of the part shown in Fig, 2, Fig. 4 shows diagrammatically an alternative arrangement of electro magnetic sound-translating device, and

Fig. 5 is a, graph illustrating the operation of farm-magnetic plate Ill and a projecting element in the form of a peg H which is driven into the mastoid hone l2 or other bone of the skull by a minor surgical operation, the armature plate l0 lying under a relatively thin layer It oi skin and tissue, a relatively thick layer of fleshy tissue l4 being interposed between the armature plate and the bone. The armature is rendered non-in- .iurious to the subject for instance by plating with silver.

For co-operation with this armature there is provided a removable magnet which is generally denoted by it. This magnet includes two pole pieces it provided with speech coils l1 and it is polarized by a permanent or electro-magnet It. The speech coils I! are included in an electric circuit i8 adapted to carry speech currents derived from any desired source. In the present example the source is controlled by means of a microphone 20 in the input circuit of an amplifier 2| which feeds the circuit l9. With sufficiently thin skin l3, no amplifier is required.

' When the magnet I5 is held (or holds itself by means of magnetic'attraction) in the position shown over the armature plate In and speech currents flow in the circuit IS, the armature cooperates with the poles I 6, owing to the interposltion'oi the resilient barrier I3 of skin and tissue,and the mechanical impulses thereby genera-ted in the armature are conveyed by the direct designed for deaf-aid or other purposes, in which corrective resonances are created.

Another object of this invention is to provide plate as close as possible to the bone, because 3 iirstly it is desirable to keep the air gap between the armature and the pole pieces as small as possible, secondly, the armature and the skull would have to be shaped to fit one another if in direct contact, for example by cambering the armature or by flattening the mastoid surface during the fitting operation, and thirdly there may be a tendency fortissue to grow between the armature and the bone and thus introduce a compliance between the armature and the bone.

The armature plate may for example be about 1 in. long, in. wide, and in. thick. In order to minimise any .tendency to thickening of the tissue defining the working gap between the ar netic properties, with the object of reducing the area of metal to be enveloped locally by tissue. Furthermore the armature may be provided with a mg in the, form of a screw-threaded stud HA adapted to be screwed into the bone.

For correcting the frequency response of a deaf person, adjustment may be made of the inherent natural frequencies of the skull itself,

, which, for the purpose of such adjustment, may

be regarded as a resonator damped through the deviation of energy into its interior, including the vibrations passing to the auditory nerves. The basic resonance frequency may be influenced by the weight and size of the armature and the peg,

their relative position and their shape and natvice ISA shown in Fig. 4, which is shown as part of a deaf-aid apparatus, a reed 22 is placed across the poles of the magnet l8, with a small leakage "gap 23, so that it diverts part of the polarising produce a magnetic shunt circuit across a portion of the main magnetic flux path through said magnet and armature and including a resilient vibrating ferro-magnetic member supported at one end and arranged to have its other end move freely and to provide an air gap in said shunt circuit, the vibrating characteristics of said member being such as to vary said air gap according to the magnetomotive force pulsations thereacrosssubstantially in proportion to the speech current pulsations in said winding, and said shunt circuit being so arranged in relation to said polarizing magnet and said speech winding that the polarity relation between the polarizing flux and the pulsating flux due to sound currents through saidwinding in the main magnetic circuit will be opposite to the polarity relationbetween the polarized and magneticiiux in said a speech winding and an armature, of means to' flux, its position being asymmetrical such that magnetic forces -tend to keep the reed under bending stress. In Fig. 5' are plotted the relationships. of the elastic force Fe and the magnetic force Fm to displacement a: of the reed. As shown in Fig. 4, the magnetic shunt constituted by the reed 22 may be between the polarising magnet II and the pole pieces ll carrying the speech coils I'I, so that the reed forms a .by-pass for the alternating flux and the leakage gap automati-. cally decreases and increases as the pulsating flux in this gap rises and falls, If the characteristics of the elastic and magnetic forces are made similar to each other, an appreciable measure of magnetic amplification may-be achieved, quite apart from the resonance eflect. the reed opening and closing the gap and acting like a valve in the magnetic shunt.

magnetic shunt circuit. 2. The combination with an electro-magnetic sound transducer comprising a polarising magnet,

produce magnetic shunt circuit across a portion of the main magnetic flux path through said magnet and armature and including an elongated resilient ferro-magnetic vibrating member supported atone end and having its other end arranged to move freely and to provide an air gap in said shunt circuit, the characteristics of the elasticforce exerted by said member and of the magnetic force thereon due to the flux through said shunt circuit being substantially alike, whereby to vary said air gap according to the magnetomotive force pulsations thereacross substantially in proportion to the speech current pulsations in said winding, and said shunt circuit being so arranged in relation to said polarizing magnet andsaid speech winding that the polarity relationbetween-the polarizing flux and the pulsating flux due to sound currents through said winding in the main magnetic circuit will be opposite to the polarity relation between the po larized and'magnetic flux in said magneticshunt circuit.

3. An electrical sound transducer comprising a polarizing magnet to provide a steady magnetic flux, a speech winding associated with said magnet to produce a pulsating flux in accordance with sound current fluctuations exciting said winding and superimposed upon said steady flux, a vibrating armature cooperating with said mag-j net, and a vibratory ferro-magnetic element secured at one point-to said magnet and having its opposite end spaced from another point of said magnet and arranged to move freely to provide a magnetic bypass including an air gap varying in accordance with the magnetic flux therethrough; said element being so arranged relative The response and 1oudness of any ordinary electro-magnetic receiver may be improved by providing the leakage reed on one side of the speech coils and the armature in the form of a reed or diaphragm on the other side,as shown in Fi 4;- both moving in even phases over certain frequency ranges below the resonancepoints and thus extending the load response downwards. lclaimz 1. The combination with an electro-magnetib sound transducer comprising a polarising magnet, a speech windingand an armature,-of means to.

said magnet to said magnet and said winding that the polarity relation between the steady and pulsating flux therethrough is opposite to the polarity relation between the steady and pulsating flux through magnetic .element having one end secured to one of the said pole pieces cantilever-fashion and having its opposite end spaced from and cooperating with the other pole piece to provide amasnetic bypass including an air' gap varying in accordance with the flux therethrough, the polarity relation between the steady and pulsating ma netic fiuxthrough said main magnetic circuit being opposite to the polarity relation between the term-magneto vibrating reed having one end secured to one of said pole pieces cantilever-fashion and having its opposite end spaced from and cooperating with the other pole piece to provide a magnetic bypass including an air gap varying in accordance with the .flux therethrough, the polarity relation between the steady and pulsating w flux through said main magnetic circuit being opposite to the polarity relation between the steady and pulsating flux through said reed, said reed having vibrating frequency characteristics of the order or the frequency 0! the sound current fluctuations.

' RUDOLF GOLDSCHMJDT. 

